This patch fixes an issue with TemplatePaths, where paths
for a given extension key get used at two execution points:

* Before TS becomes available
* At any time after TS becomes available

In this case, TemplatePaths holds on to the fallback paths
which the first rendering detects (since TS is not availble,
fallback paths get returned) and all subsequent rendering
of any template in that extension context, yields only the
fallback paths and ignores what is defined in TS.

By not allowing fallback paths to be cached in runtime, the
problem is prevented.